The present invention relates to semiconductor devices and manufacturing methods of semiconductor devices, and more particularly, to a technique effectively applied to a semiconductor device having a nonvolatile memory.
Flash memories are widely used as one type of an electrically erasable and programmable read only memory (EEPROM) which is an electrically writable and erasable nonvolatile semiconductor memory. The flash memory includes a conductive floating gate electrode or a trap insulating film enclosed by an oxide film under the gate electrode of a metal-insulator-semiconductor field-effect transistor (MISFET). The flash memory is designed to store information therein using a difference in threshold of the MISFET between the presence and absence of charges (electrons or holes) in the floating gate or trap insulating film.
For example, Japanese Unexamined Patent Publication No. 2006-303918 (Patent Document 1) discloses a technique for a MONOS-type nonvolatile memory in which a memory gate electrode is formed of a doped polycrystal silicon film consisting of two layers, namely, a high-concentration lower layer and a low-concentration upper layer.
Japanese Unexamined Patent Publication No. 2006-19373 (Patent Document 2) discloses a technique for a MONOS-type nonvolatile memory in which a memory gate is formed of a doped polycrystal silicon film so as to have a sheet resistance lower than that of a control gate formed of a polycrystal silicon film by ion-implanting impurities into an undoped silicon film.
Japanese Unexamined Patent Publication. No. 2004-186452 (Patent Document 3) discloses a technique for a MONOS-type nonvolatile memory which involves depositing a polycrystal silicon film doped with n-type impurities after forming a selection gate electrode, ion-implanting p-type impurities into a semiconductor substrate in this state, then depositing thereon another polycrystal silicon film doped with n-type impurities, and performing anisotropic etching on the two n-type impurity layers deposited to form a memory gate.